The process of the invention relates to the separation and recovery of alkoxysilanes, particularly alkyldialkoxysilanes, from crude reaction mixtures obtained from the Direct Reaction of silicon with alkanols. More particularly, this invention is directed to the membrane separation and recovery of methyldimethoxysilane (CH3SiH(OCH3)2) from a crude reaction mixture obtained from the Direct Reaction of silicon with methanol.
The Direct Reaction Process, as it will be referred to herein, of silicon metal with an alkanol to form trialkoxysilanes has achieved commercial prominence especially where methanol is the alcohol and the product is trimethoxysilane (see, e.g., Chemical Engineering, November 1999, pp. 92-93). Despite the commercial success of this process, practical problems still exist, the most important of which is that the methanol/silicon reaction is incomplete and the product stream exiting the reaction zone is primarily a mixture of unreacted methanol, trimethoxysilane product and tetramethoxysilane by-product together with minor amounts of other co-products at much lower levels. A number of problems attend the processing and separation of the product stream. Firstly, unreacted methanol and trimethoxysilane product form a low-boiling azeotrope consisting of a nearly 3/1 molar ratio of methanol to trimethoxysilane. Secondly, methanol and trimethoxysilane react with each other to form tetramethoxysilane and hydrogen gas, and can do so violently if the self-accelerating decomposition temperature of the azeotrope is exceeded, or if the azeotrope contacts certain catalytic contaminants. Thirdly, one of the minor co-products, namely, methyldimethoxysilane, has utility and economic value apart from trimethoxysilane for making a variety of silane coupling agents or intermediates possessing only two methoxy groups bonded to the silicon atom. Isolation of methyldimethoxysilane is hampered not only by the aforementioned methanol/trimethoxysilane azeotrope but also by the occurrence of a methanol/methyldimethoxysilane azeotrope.
Processes have been developed to deal with these azeotropes. For example, solvents have been added to the azeotropes either to form new, even lower-boiling azeotropes or to extract product away from methanol. The former route employing hexane is disclosed in JP 61/039955 (Chem. Abstr., 106, 33302 k(1987)), and the latter employing polydimethylsiloxane is disclosed in JP 60/252488 (Chem. Abstr., 104, 148307s(1986)). Similarly, in U.S. Pat. No. 4,761,492, an extractive distillation using tetramethoxysilane is disclosed. These processes all involve the handling of significant quantities of solvents and are therefore undesirable from an economic viewpoint. They also lead to a less pure grade and lower yield of trimethoxysilane due to the more extensive distillations that are required for their removal.
Most of these difficulties are avoided by the process disclosed in U.S. Pat. No. 4,999,446 wherein the cited azeotropes are recycled directly to the reaction zone thereby enabling continuous partial recovery of the trimethoxysilane portion of the methanol/trimethoxysilane azeotrope while the methanol portion undergoes further reaction with silicon metal. In this mode, the possibility of undesired reaction of methanol with trimethoxysilane is no greater than that during normal operation of the reactor and is controlled by adjusting the total flow of fresh and recycled methanol to the reactor.
More recently, U.S. Pat. No. 6,255,514 discloses treatment of the methanol/trimethoxysilane azeotrope with a salt in the optional presence of a solvent. However, there is still the problem of handling significant quantities of solvent compounded by the problem of adding a solid salt. A similar method of purifying alkoxysilanes other than trimethoxysilane is disclosed in U.S. Pat. No. 6,861,546.
The ethanol/silicon Direct Reaction Process has also achieved some level of commercial success. However, ethanol-containing azeotropes have not been observed in the reactor effluent. Nevertheless, the formation of ethyldiethoxysilane, corresponding to methyldimethoxysilane in the methanol Direct Reaction Process, does occur.
Although, in current commercial practice, the respective alkyldialkoxysilanes are usually present at low levels in the crude reaction mixture (typically less than 5 weight percent), processes are known whereby the yield of alkyldialkoxysilane can be increased. For example, U.S. Pat. No. 4,778,910 describes the reaction of methanol with copper-silicon alloy in the presence of an alkali metal co-catalyst (for example, potassium formate) under autogenous conditions at about 200-400° C. to produce a methoxysilane mixture containing about 8-9 weight percent methyldimethoxysilane. Accordingly, separation and recovery of alkyldialkoxysilanes from trialkoxysilanes and tetraalkoxysilanes and unreacted alkanol in the reactor effluent of the alkanol/silicon Direct Reaction Process is both desirable and necessary, even when azeotropes are not formed.
Thus, there is a continuing need for a process which, for example, will separate a methanol/trimethoxysilane azeotrope into its components, or at least into two fractions richer in each respective component than the original azeotrope, said process being continuous in nature, optionally with recycle of either fraction to the reactor or to the distillation column, for improved separation. Such a process should keep the methyldimethoxysilane co-product with the enriched trimethoxysilane fraction such that the co-product can be further enriched (for example, by distillation) and isolated as a separate product. In addition, there is a continuing need for an alkanol/silicon Direct Reaction Process which will provide higher yields of alkyldialkoxysilanes, the isolation of which will be enhanced by the aforesaid separation process.
A membrane is a barrier, which permits one or more components of a mixture to selectively permeate it thereby changing the composition of the fluid stream traversing it. Molecular size, molecular mass and cohesive energy density (solubility parameter) are commonly the bases of separation and the driving force for the separation can be pressure, concentration or electric potential gradient. The rate of transport (permeability) of the components and the selective permeation of components are the most important functional characteristics of a membrane. These characteristics are combined quantitatively in the permselectivity property of the membrane, defined as the ratio of the permeabilities of a component and a reference (standard). Permselectivity is the most distinctive property of a membrane. Components of higher permselectivity become enriched on the permeate side of the membrane relative to their concentrations in the feed composition.
Membranes are obtained from a variety of polymeric and inorganic materials. Examples include silicones, polysulphones, polycarbonates, polytetrafluoroethylene, nylon, silica, stainless steel, palladium, silver, alumina and zirconia. Membranes can be constructed as sheets, hollow fibers, spirals and tubes to maximize surface area/volume ratio. Comprehensive descriptions of the state-of-the-art in membrane materials, configuration, classification and applications can be found in the following publications: H. K. Lonsdale, J. Membrane Sci., 10(1982) pp 81-181; J. A. Howell, “The Membrane Alternative Energy Implications for Industry”, Watt Committee Report Number 21, Elsevier Applied Science, London (1990); G. Saracco and V. Specchia, Catalysis Reviews—Science & Engineering, 36(1994) pp 305-384; Catalysis Today, 25 Nos. 3 and 4 (1995), pp. 197-291; A. Tavolaro and E. Drioli, Advanced Materials, 11(1999), pp. 975-996; and, M. A. Mazid and T. Matsuura, Separation Science and Technology, 28(1993) pp. 2287-2296.
Application of separation processes which involve the use of porous and/or dense semi-permeable membranes for separating compounds can save in process costs because energy consumption is low, raw materials and intermediates can be recovered and reused. When the feed is in liquid state, the separation processes using membrane technology include nanofiltration, reverse osmosis, pervaporation, perstraction, and electrodialysis. When the feed is in gas or vapor phase, the separation processes using membrane technology include vapor permeation and gas permeation.
Nanofiltration and reverse osmosis involve feeding a liquid mixture on one side of a membrane at high operating pressures, while maintaining the system on the opposite side of the membrane at atmospheric pressure. Thus, the resulting permeate remains in the liquid phase. Conventional nanofiltration and reverse osmosis membranes are fabricated from cellulose derivatives and interfacial polyamide thin film composites. A disadvantage of the reverse osmosis and nanofiltration process employing conventional membranes is that the highest concentration of the liquid mixture that can be obtained is about twenty percent due to the high osmotic pressure requirements.
The pervaporation process involves feeding a liquid mixture on one side of a membrane at or near atmosphere pressure, while maintaining the system on the opposite side of the membrane at a sufficiently low vapor pressure to vaporize the liquid component(s). The resulting permeate traverses the membrane as a vapor and is collected either in its gaseous state or recovered by condensation, adsorption or any other suitable method. Instead of a vacuum on the downstream side of the membrane, a sweep gas can be used to remove the permeated product. In this mode of operation, the permeate side is at atmospheric pressure. Vapor permeation differs from pervaporation in that the feed is already in the vapor phase.
The advantages of pervaporation and vapor permeation processes are that they are applicable to the separation of azeotropic mixtures that cannot be separated by an ordinary distillation, or to the separation of a mixture of compounds having close boiling points, or to the concentration of a compound which is sensitive to heat, or to the separation of isomers. Moreover, unlike reverse osmosis, these separations or concentrations are applicable over the entire concentration range that is to be treated.
In a perstraction process, the permeate molecules in the feed dissolve into the membrane film, diffuse through the film and reemerge on the permeate side under the influence of a concentration gradient. A sweep flow of liquid is used on the permeate side of the membrane to maintain the concentration gradient driving force.
Hagerbaumer et al., AICHE Chemical Engineering Progress, Symposium Series 10, 50(1954), pp. 25-50 disclose the use of membranes for the separation of azeotropes. However, no mention is made in this publication of the application of membrane technology to the separation and recovery of silanes and/or silicones. Additionally, membranes have found widespread utility for water/alcohol separation, air separation, hydrogen recovery and the separation and recovery of a wide range of organic compounds and drugs.
U.S. Pat. No. 4,941,893 and Hsieh et al., J. Membrane Sci, 70(1992) pp 143-152 both disclose the separation of monosilane (SiH4) and halosilanes from hydrogen and hydrogen halides using polysulfone membranes. WO 2002/070112 discloses the use of hydrophobic pervaporation membranes (for example, a composite polyvinyledene membrane coated with silicone rubber) for the separation of cyclic siloxanes from aqueous silicone emulsions. None of these publications describing membrane separation of silicon compounds deals with the separation and recovery of alkyldialkoxy-silanes.
Alkyldialkoxysilanes such as methyldimethoxysilane and ethyldiethoxysilane are useful raw materials for the hydrosilylation of unsaturated substrates to prepare organofunctional silanes used in coatings and surface modification. Examples of these organofunctional silanes are methylvinyldimethoxysilane, gamma-aminopropylmethyl-dimethoxysilane and glycidoxypropylethyldiethoxysilane. Methyldimethoxysilane and ethyldiethoxysilane are also desirable as starting materials for plasma-enhanced chemical vapor deposition of low dielectric constant silicate coatings on silicon wafers.